This is the only mechanical switch keyboard with bluetooth that I've been able to find on the entire internet (and in fact they claim to be the first, so it might be the only only one). Looks like it fills all your needs except solar powered.

If you're looking for something like the Das, the KBtalKing 1.1 is up for preorder with the same Cherry MX Blue (clicky) and MX Brown (non-clicky) switches that the Das keyboard use. The 1.0 is MX Reds (similar to the Browns, but with linear key travel instead of a tactile engagement point). And none of the Cherry switches are buckling-spring, so I'm not sure where you got that idea.

It's not technically buckling spring, but the Cherry MX Red switches are mechanical. The Das Keyboard uses Cherry MX Blue switches, which arguably simulate the buckling spring action better-- more of an audible click. Reds are a bit away from that end of the spectrum with less of a physical and audible click-- they have an action that's straight down without variable force.

Man, do your own research at this point. It took me literally 2 minutes on Google to find what I did with no pre-knowledge whatsoever. You should be able to find non-mechanical switch bluetooth keyboards pretty easily, since that's literally every other bluetooth keyboard that exists.

How are you going to use multi-pairing?Would for example a single pairing and free softwares like "Microsoft Garage: Mouse without borders" (windows only but all windows versions) and/or "Synergy"/"Synergy plus" (Linux, Unix, Windows, Mac), solve the linking problem?

Not knowing how you intend to use it limits the range of solutions we can come up with, so I was just throwing this in to illustrate that the solution you envision to your problem may not be the only way, if we just knew what you want to accomplish.